Introduction
Duckweeds are small, floating aquatic plants with a simplistic
morphology. Individual duckweeds consist of single fronds with zero to
multiple roots attached to the bottom surface. Duckweeds can flower;
however, they rarely do so (Hicks, 1932). Their main reproduction
strategy consists of asexual budding, where several daughter fronds bud
and then detach from the mother frond (Laird & Barks, 2018). An
individual frond can produce up to a couple dozen of daughter fronds
over its life, which is only a few weeks short. Duckweeds are found
around the world in slow flowing freshwater systems, when suitable
anchoring possibilities are present (Landolt, 1986). Their fast
reproduction cycle and their short life span make duckweeds useful model
organisms for research in ecology and evolution (Laird & Barks, 2018).
Duckweeds can tolerate high levels of nitrogen, phosphorus and heavy
metals and different species can have different responses to
temperature, light, nutrients and toxicants (Landolt, 1996). In nature,
duckweed species frequently co-exist (Landolt, 1986).
Zn is an essential trace element for plant growth, but elevated
concentrations inhibit growth and can lead to chlorosis. Therefore,
elevated Zn levels are phytotoxic (Rout & Das, 2009). Zn is a commonly
used building material and through run-off from roofs, galvanized items,
and pipes it finds its way into (underground) waters, leading to Zn
pollution (AWEL, 2006). In Switzerland, between 2006 and 2014, Zn
concentration measurements exceeded the indicator value of 5 µg/l at 15
measuring stations, more than any other trace element measured
(Bundesamt für Umwelt BAFU, 2019).
In the face of pollution of water systems, Lemnaceae are studied
as potential organisms for phytoremediation (Liu et al., 2021). For
example, one species of duckweed, Lemna minor L. (common
duckweed), has shown to be a good accumulator of heavy metals such as
Cadmium, Selenium and Copper (Zayed et al., 1998). Several studies have
shown metal accumulation in different duckweed species (Lahive,
O’Callaghan, et al., 2011), which depended both on the species (Cardwell
et al., 2002) and on the metal (Gaur et al., 1994). A prerequisite for
metal accumulation, however, is the tolerance of a species to elevated
levels of heavy metals. Duckweed species differ in their tolerance to
Zn: L. minor was shown to tolerate Zn concentrations above 100
mg/L but the gibbous duckweed Lemna gibba only tolerated
concentrations up to 10 mg/L (Lahive, O’ Halloran, et al., 2011). Zn
tolerance of other duckweed species such as Lemna turionifera(red duckweed) was, to our knowledge, never investigated.
Additionally, there is a lack of research concerning the influence of
species interaction on duckweed resistance to metal pollution. Previous
studies suggest that species interactions in duckweeds can influence
growth rates (Clatworthy & Harper, 1962; Gopal & Goel, 1993; Peeters
et al., 2016). Here, we hypothesized that the presence of a second
species in a mixed setting could increase heavy metal tolerance because
of facilitation. The stress-gradient hypothesis predicts that
interactions among plants are context dependent, shifting from
competition to facilitation as environmental stress increases (Callaway,
1995). At high Zn concentrations, if the more tolerant duckweed species
accumulates Zn present in the medium, this could facilitate the
persistence or even growth of the co-occurring, less heavy-metal
tolerant species. However, at low levels of Zn concentration,
competition for nutrients could override any facilitative mechanisms,
leading to a negative effect of the presence of a second species.
To test our hypothesis, we grew three Lemnaceae species in
isolation and in two-species pairings along a zinc sulfate
(ZnSO4) concentration gradient (0, 0.45, 1.82, 11.35
mg/L Zn). We measured the Zn tolerance of three duckweeds speciesL. minor, Lemna gibba, and L. turionifera over 17 days in
replicated microcosms under sterile and controlled conditions.